Java npe when adding a partition which already exists

create external table if not exists at (
id string, bool_col string, i string, s string, ic string, bi string, f string, d string, da string, sc string, ts string) 
partitioned by (year int, month int) 
location '/test-warehouse/alltypesrror';

alter table at add if not exists partition (year=2000, month=1);

E0524 10:28:53.734863 29263 ImpaladCatalog.java:134] Error adding catalog object: null
Java exception follows:
java.lang.NullPointerException
	at com.cloudera.impala.catalog.ImpaladCatalog.addTable(ImpaladCatalog.java:350)
	at com.cloudera.impala.catalog.ImpaladCatalog.addCatalogObject(ImpaladCatalog.java:246)
	at com.cloudera.impala.catalog.ImpaladCatalog.updateCatalog(ImpaladCatalog.java:132)
	at com.cloudera.impala.service.Frontend.updateCatalogCache(Frontend.java:227)
	at com.cloudera.impala.service.JniFrontend.updateCatalogCache(JniFrontend.java:161)